The present invention relates to a process for decolorizing aqueous sugar solutions with an adsorbent resin prepared under suspension polymerization conditions commonly used for ion exchange resins.
Decolorization is a key process in sugar refining. Traditionally, aqueous sugar solutions are decolorized with carbon adsorbents, such as bone char or granular and powdered activated carbon. Unfortunately, the use of carbon has some negative features In decolorizing with activated carbon, the equipment used in the process is expensive, difficult to operate and to maintain. Another negative feature of carbon is that carbon is abrasive so it is hard on the equipment employed, thereby adding to the replacement cost of the equipment. The fuel for thermal regeneration and carbon replacement of burned carbon are also expensive.
Ion exchange resins have been used to demineralize and decolorize sugar solutions. Unfortunately, the capacity of conventional ion exchange resins for adsorbing color bodies from solution is low relative to the adsorptive capacity of carbon, and the physical stability of some resins used for decolorization is poor. Further, loaded resins require regeneration with strong acids, strong bases, or concentrated salt solutions, which frequently do not remove a substantial quantity of the adsorbed components from the resins, and therefore such resins are prone to fouling during use. These resin characteristics significantly increase the costs associated with using ion exchange resins for decolorizing aqueous sugar solutions, and thus detracts from their commercial viability.
As examples of some of the deficiencies associated with the use of ion exchange resins for decolorizing sugar solutions, the following patents, which describe such processes are discussed and analyzed, U.S. Pat. No. 3,122,456: U.S. Pat. No. 2,578,938: and U.S. Pat. No. 4,193,817.
U.S. Pat. No. 3,122,456 discloses a method of purifying and decolorizing sugar solutions with "spongy" synthetic ion exchange resins. The resins are prepared from crosslinked copolymers permeated by small cavities or veins. Unfortunately, not only do these spongy resins exhibit an adsorptive capacity less than the capacity of carbon, but they also swell considerably upon contact with aqueous solutions - from 200 to 500 percent of their original dry volume. Thus, the resins have very poor physical stability, particularly during acid and base cycling which would be necessary to decolorize a sugar solution and to regenerate the resin. The high swellability and the poor physical stability of the resins additionally increases their tendency to foul during regeneration.
U.S Pat. No. 2,578,938 discloses a method of decolorizing sucrose solutions with a lightly crosslinked ion exchange resin. Unfortunately, this resin exhibits the same characteristics associated with the spongy resins, and therefore represents a poor alternative to carbon for decolorizing sugar solutions.
U.S. Pat. No. 4,193,817 discloses decolorizing a bottler's sugar solution with a strong base anion exchange resin. Once again the adsorptive capacity of the resin is poor relative to that of carbon. The resin is regenerated by first contacting the resin with an aqueous solution of sodium chloride and either an alkali metal or alkaline earth metal hydroxide (required to convert resin to hydroxide ion form) and then contacting the resin with an aqueous solution of either sodium chloride or hydrochloric acid (required to convert resin to the desired chloride ion form for subsequent decolorization of sugar solution). Unfortunately, the high dry weight capacity of the resin requires using large amounts of base or acid regenerants, which increase regeneration cost.
In view of the deficiencies of the prior art, a process for decolorizing sugar solutions with an adsorbent having an adsorptive capacity similar to that of carbon is needed. Furthermore, a process employing an effective adsorbent with good physical properties that can be easily and cost-effectively regenerated is needed.